This invention relates to a method and apparatus for making acoustical tile utilized primarily in ceiling construction. In particular, the method and apparatus for this invention produce an improved cast ceiling tile which has uniformity of density.
While a large variety of formulations may be used, cast ceiling tiles are generally made with a combination of fiber material and a binder, preferably a starch binder. An example of a typical prior art process is shown and described in U.S. Pat. No. 3,246,063 (the '063 patent). The '063 patent describes a process in which a composition of granulated mineral wool and a binder is deposited in a tray which has been lined with a foil sheet. The binder of the '063 patent is an amylaceous starch which, when mixed with water and mineral granulated wool, is placed on a tray in a layer. The composition is subsequently leveled with a reciprocating screed bar. The composition is then oven-dried into slabs and cut into tiles.
A substantial difficulty with the process shown in the '063 patent relates to the density of the final product. Density is an important consideration from the standpoint of structural integrity and strength, and because of thermal and acoustical considerations. The problem of achieving a uniform density relates to the manner in which the uncured composition is deposited in trays. A quantity of fluid uncured mixture is poured into a box which has an open bottom. Trays are placed on a conveyor and moved horizontally under the box. Generally, the opening of the bottom of the box is approximately the same width as the tray. When the tray moves past the opening in the box, the fluidized mixture or pulp fills the tray, and one edge of the box scrapes the surface of the filled tray to a given height. However, at the outside edges of the tray, the flow of pulp is inhibited by frictional contact with the sides of the box which are parallel to the direction of movement of the tray. The slower flow of pulp at the edges creates openings or fissures in the pulp as the tray moves out from under the box. Such fissures and open areas tend to weaken the outer edges of the tiles. The resulting inconsistencies in density have consequences which relate to the machinability, as well as the appearance of the tiles. Inconsistency in tile density may also have consequences relating to the porosity of the tile, which may be important in applications where ventilation systems rely on the tile material to direct air flow.
A wide variety of formulations can be used to manufacture starch-based ceiling tiles. Consistency of the tile material is extremely important, primarily because the tiles must have a uniform surface texture. Even minor variations in surface texture may be obvious from tile to tile, making a ceiling unattractive.
It is an object of the present invention to provide a method for producing ceiling tiles which have uniform density.
It is another object of the present invention to provide an apparatus for making ceiling tiles with uniform density.
It is a further object of the present invention to provide a method for making ceiling tiles with uniform surface texture.
Another object of the present invention is to provide an apparatus for making ceiling tiles which have uniform surface texture.
Yet another object of the present invention is to provide a machine and method for depositing a layer of pulp so that when it is shaped and subsequently rolled with a roller, the layer has a substantially uniform density.
Still a further object of the present invention is to provide a ceiling tile which has uniformity of both density and texture.
These and other objects of the present invention are achieved with an apparatus in which a conveyor is used to carry a series of trays underneath a pulp feeder box. The trays may, or may not, be lined with a flexible backing. As the trays move underneath the feeder box, pulp comprised of an aqueous mixture of starch and fibrous material is deposited in the trays. Because the upper exposed surface of the pulp layer will eventually be the visible surface of the ceiling tile, formation of the pulp surface layer is critical. In the apparatus of the present invention, the layer is deposited in the trays in an uneven configuration with outer edges being thicker than inner portions of the layer. This uneven layer is formed with a curved edge on the bottom of the feeder box. A roller is then used to level the layer, providing it with a substantially uniform density and surface texture. The slabs are then hardened by baking. The hardened slabs are then cut and finished in accordance with known techniques. It should be noted that the byproducts of finishing the slabs into tiles can be used and reclaimed by including them in subsequent batches of pulp.